A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture from about 500 to 1000 times its own weight. Various manufacturers have denominated it as different names, such as SAM (Super Absorbency Material), AGM (Absorbent Gel Material), etc. Since such superabsorbent polymers started to be practically applied in sanitary products, now they have been widely used not only for hygiene products such as disposable diapers for children, sanitary napkins, etc., but also for water retaining soil products for gardening, water stop materials for the civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultice, etc.
In most cases, these superabsorbent polymers have been widely used in the field of hygienic materials such as diapers, sanitary napkins, etc. For these applications, superabsorbent polymers are required to exhibit high absorbency with respect to moisture, etc., must not release absorbed water even under an external pressure, and also must maintain the shape under volume expansion (swelling) due to water absorption to show excellent permeability.
Reportedly, it is difficult to improve centrifuge retention capacity (CRC), which is a basic physical property of showing water absorption and retention capacities of the superabsorbent polymer, and absorbency under load (AUL), which is a property of retaining absorbed water even under an external pressure, at the same time. The reason is that when the overall crosslinking density of the superabsorbent polymer is controlled to be low, centrifuge retention capacity becomes relatively high, but a crosslinking structure becomes loose and gel strength becomes low, leading to a reduction in absorbency under load. On the contrary, when the crosslinking density is controlled to be high, and therefore absorbency under load is improved, water is hardly absorbed between compact crosslinking structures, leading to a reduction in basic centrifuge retention capacity. Because of the above-described reasons, there have been limitations in providing superabsorbent polymers in which centrifuge retention capacity and absorbency under load are improved at the same time.
However, superabsorbent polymers have been required to have higher absorption performances with recent slimness of sanitary materials such as diapers, sanitary napkins, etc. Of them, simultaneous enhancement of centrifuge retention capacity and absorbency under load which are incompatible physical properties, and improvement of liquid permeability are emerging as important issues.
Further, a pressure by a user's weight may be applied to sanitary materials such as diapers, sanitary napkins, etc. In particular, when liquid is absorbed by the superabsorbent polymer used in sanitary materials such as diapers, sanitary napkins, etc., and then a pressure by a user's weight is applied thereto, a rewetting phenomenon may occur, in which the rewetting phenomenon causes the superabsorbent polymer to release part of the absorbed liquid again. Therefore, to avoid this rewetting phenomenon, many attempts have been made to improve absorbency under load, liquid permeability, etc. However, a specific method capable of effectively avoiding the rewetting phenomenon has not been suggested yet.